Alum in this sense is pronounced like the first two syllables of alumnus/alumna: /əˈlʌm/ uh-LUM, with stress on the second syllable. It is not pronounced the same way as alum the chemical compound, which has stress on the first syllable: /ˈæləm/ AL-um. They are different words that happen to be homographs.
The Oxford English dictionary says
Originally in nonstandard use, or as a graphic abbreviation. Now usually as a gender-neutral alternative to alumnus n. or alumna n.
The first two citations show a spelling with n; the second one has a period, fitting with the description of the form as a graphic abbreviation.
1877 tr. J. H. Balfour in Upsala Universitets Fyrahundraårs Jubelfest (1879) 49 The earnest hope, that..Sweden may continue to send forth many alumns who shall do credit to her great Educational Institutions.
1894 Alumni Rep. (Philadelphia Coll. Pharm. Alumni Assoc.) 132/2 The establishment of a microscopical laboratory..the members of the Executive Board of the Alumn. Association..would strongly favor the same.
However, by the 20th century, the OED quotations include forms spelled without n that look more like phonetic clippings that would actually be used in pronunciation:
1928 Chicago Daily Tribune 13 Dec. 37/1 The local Harvard ‘alums’ have a number of parties in the incipient stage of planning.
1974 State (Columbia, S. Carolina) 15 Feb. 1- b/1 Streaking is darting out (unexpectedly) in the nude.., preferably..in front of a carload of staid old alums.
Latin has a number of words that change form based on their gender. For words like alumnus, aluma, there is no Latin form that is explicitly gender-neutral or unspecified for gender. The masculine singular form (alumnus) can be found used in some cases for an unspecified individual, and the masculine plural (alumni) for a mixed or unspecified group; the male-specific and generic uses of the masculine form can only be distinguished by context. The generic use of masculine-gender nouns has been argued to be sexist, so many organizations now avoid it. This avoidance is not based on ignorance of the generic use of masculine forms, but on an objection to this usage.
Latin has a neuter gender, but this is not the same thing as a gender-neutral form. Latin words of the neuter gender are not used to refer to a person of unspecified gender any more than it in English is used to refer to a person of unspecified gender. So the use of alumnum in the sense "an alumnus or alumna" is not precedented in Latin.